Original subtitles

You're speeding though space 60,000 miles an hour.

Precisely a year from now,

you will have travelled 584 million miles - to end up exactly where you started.

In 365 clays, all of us on planet Earth make this epic journey once around the sun.

But because of the orbit takes around its star,

the way our planet spins, and the way it's tilted over, our journey brings with it extreme weather.

Did you see that'? The whole house blew apart.

It produces bizarre phenomena.

And drives earth's diverse climates.

The connection between the Earth and the Sun defines our planet.

It's changed the course of human history.

It's responsible for the story of life itself.

Our lives are divided into 24-hour periods.

For some of that time it's dark, and for the rest of the time it's light.

Day and night are easily explained by the fact that the Earth spins on its axis as it orbits the sun.

But what happens in the Arctic shows that things aren't quite so simple.

Because here in the summer time there is only day.

The sun never sets at all.

And it never sets because of something that happened to our planet...

...four and a half billion years ago.

When our solar system began, it was a crowded and dangerous place.

Earth was just one of thousands of new planets orbiting the young Sun.

And it was on a collision course with another, smaller planet called Thea.

Thea was totally destroyed.

Debris from the crash eventually formed our Moon.

Earth survived, but the crash had changed it forever.

Thea knocked the Earth over.

Our planet spins around its axis at an angle of 23 degrees away from the upright.

This 23-degree tilt means that as Earth orbits the Sun,

different parts of it get more or less exposure to the Sun.

In June and July,

the tilt means that the northern hemisphere is facing towards the Sun.

It's summertime.

But in the south, the opposite is true.

The southern hemisphere is getting far less sunlight.

It's the southern winter.

Comem December and January, the Earth is on the opposite side of the Sun,

so the tilt means it's winter in the north and summer in the south.

At the poles, the sun never sets in summer and never rises in winter.

But it's not just at the extreme ends of the planet that the tilt is significant.

For the rest of us in between, the length of our days changes through the year,

because of the Earth's tilt.

Tilt brings us spring, summer, winter an fall.

Without it, there would be no seasons.

As the seasons change up and down the surface of the Earth,

they bring on dramatic and spectacular events.

From the intense storms of Tornado Alley.

We're watching some very strong rotation here.

This purple area here is winds about 150 kilometres towards the radar.

To the dramatic spring break-up of ice on frozen rivers.

L've been watching it for 62 years.

You're going to be faced with high water and river flow ice.

You've got nothing at all except a flood.

From the mass migration of Arctic animals.

To the Indian subcontinent's monsoon.

This is all the water that's been lifted up off the ocean in the past few clays,

because of the heat and been carried inland and is now falling on top of me.

All these phenomena are a direct result of the Earth's 23-degree tilt.

It's March 20th, and crowds of people are gathering in the ancient Mayan city of Chichen ltza in Mexico.

They've come to witness an event that's inextricably linked to the tilt of the Earth.

And event so significant, this, the great pyramid of Kukulkan,

was built 1200 years ago to capture it.

It doesn't take much to work out that the temple has something to do with the year.

There are four sides which represent the four seasons.

Each has 91 steps.

These, combined with the top platform, make a total of 365 -

one for every day of the year.

But the temple is more than an ancient stone calendar.

It also records the solstices, the shortest and longest days of the year,

the extremes of the Earth's tilt relative to the sun.

And it records the spring and fall equinoxes,

when for a moment there is no tilt relative to the Sun.

Today is the spring equinox,

a point in the year when day and night are of equal duration.

The crowds at Chichen ltza are waiting for a magical transformation...

...which only happens here on the day of the equinox.

As the afternoon sun aligns with the pyramid, it casts a shadow along the edge of this staircase.

As it develops, the shadow provides the carved snake's head at the bottom of the steps with a body.

This is Kukulkan, a heavenly serpent, divine messenger of the Mayan sun god.

The ancient Mayans realised that the equinox was a significant point in the year,

and understood enough about the seasons to capture this moment in stone.

But they didn't know why it was significant.

They didn't know that this is the point in its orbit at which the Earth's tilt is irrelevant.

Because it's axis is momentarily at right angles to the Sun,

neither pole is tipped towards it,

and so everywhere on Earth receives the same amount of night and day.

From this clay forward, the Earth's 23-degree tilt will reassert itself.

As the northern hemisphere tilts more and more towards the Sun,

the days become longer and warmer.

Unsurprisingly, this turning point has been traditionally marked as a time for renewal.

From the pagan dawn ceremony at Stonehenge -

to the burning of the wicker man in Northern Ireland.

And that Catholic fiesta de las Fallas in Valencia, Spain.

As the northern hemisphere tilts further towards the Sun,

the extra heat and light have a dramatic effect on animals and plants.

Longer spring days trigger breeding cycles by stimulating reproductive hormones,

and stronger sun also stimulates trees and plants to grow,

and the green creeps north as the global tilt increases.

Winter has covered the far north in snow,

and now it is melting back towards the Arctic circle.

Spring is about to arrive in dramatic fashion.

This is the Hay river, which flows for hundreds of miles through northern Canada.

At its mouth, bordering Great Slave Lake, lies the town of Hay River.

In winter, both lake and river are covered by a thick layer of ice.

In spring, as the Earth's tilt brings the Sun northward,

the frozen river starts to melt and break up.

It's an annual event that former Hay River mayor, Red McBryan, has witnessed for most of his life.

L've been watching it for 62 years.

You're going to be faced with high water and river flow ice.

You've got nothing at all except a flood.

Flooding happens because the river flows from the warmer south to the colder north.

Great Slave Lake is still frozen solid when blocks of melting ice arrive from upstream to the south.

This forms ice dams which can force the river over its banks, flooding the town.

The trouble is, though, no one knows if the flood will happen or not.

Which is why Jennifer Nafziger...

...has been brought in to take some of the uncertainty out of the Hay River spring.

We're trying to help the town of Hay River understand how much flooding...

...that they might have in town and when that flooding might occur.

Jennifer and her team are using time lapse photography...

...to monitor the break-up of the river ice.

In particular, they're interested in this waterfall 30 miles upriver from the town.

When ice starts to flow from the river over these falls,

that's the indication for the town that break-up is going to happen,

and that's when the town should be starting to do their flood watch operation.

This year when the falls were breached the town escaped unscathed.

Jennifer hopes that the data she and her team...

...have gathered will enable future accurate prediction of flooding.

But it'll take more data and more time.

After all, she's up against a whole chain of events that started four and a half billion years ago,

when the Earth's 23-degree tilt was born.

Next year it may be all hell to pay.

It may be two feet thick, higher water,

and she'll just go through here and - and play hell with us.

As spring progresses towards summer, the Earth's tilt begins to thaw the Arctic itself.

Above the Hay River, caribou are on the move,

following the sun north towards the tundra, newly free of snow.

Pregnant females arrive first and deliver their calves.

Within a couple of weeks, the rest of the herd gathers.

Caribou herds across the north will feed and fatten up during the endless daylight of Arctic summer.

At the other end of the planet, the opposite seasonal change is under way.

As the Earth's tilt draws the southern hemisphere farther and farther away from the sun,

temperatures plummet and the days shorten.

In Antarctica, another epic migration is under way.

Humpback whales have been feeding here during Antarctic summer.

Now, with the approach of winter, they head north...

...to warmer breeding grounds off the coast of Australia.

But as some species abandon the Antarctic's punishing winter,

others use it to their advantage.

The days are much shorter and colder now in the southern hemisphere.

It's a perfect time for emperor penguins to mate,

because the growing sea ice is finally strong enough to support their vast breeding colonies.

And it provides a place of relative safety for the young.

All over the planet,

the natural world reacts to the Earth's changing relationship to the Sun...

...as its 23-degree tilt becomes more or less significant.

As spring turns to summer in the northern hemisphere,

the sun's rays warm the earth producing thermals,

the updrafts of warm air beloved of para-glider pilots.

We're finally getting above launch height.

Better check our altitude.

We're 2300 metres.

Let's see how high we can take this one.

Paragliding champion Hunter Ramanek is no ordinary birdman.

He's also a meteorologist, so understands the atmosphere better than most.

Hunter's great skill...

...is to ride the complex currents of air stirred up by the Rocky Mountains beneath him.

At this time of the year,

solar-powered thermals are another force he can exploit to keep airborne.

Thermals exist because the ground has been heated by the sun,

and the ground in turn heats the air that overlies it.

A thermal is a warm rising plume of air that is invisible,

but is ascending upwards at sometimes several hundred feet per minute.

Hunter is exploiting the fundamental principle of atmospheric physics - that hot air rises.

When air warms and expands, it becomes less dense.

As it floats upwards, it forms a rising thermal column.

It's a very powerful feeling to be spiralling skyward on something that's just created by the sun.

But thermals do a lot more than just keep paraglider pilots happy.

As they rise, thermals eventually mix with the cooler air above.

Often the rising warm air carries water vapour with it.

When it hits the colder air aloft,

the water vapour is transformed into a mass of tiny droplets and a cloud is formed.

This basic process lies at the heart of every significant weather event on the planet.

It's the tilt of the Earth that adds intensity to this thermal activity,

especially as it changes the seasons.

In springtime up and down the American Midwest,

there's a regional phenomenon that demonstrates the thermal process in a dramatic way.

Spring brings warm, wet air masses from the south,

creating the perfect conditions for one of the planet's most severe weather events.

It's why this part of the Midwest is called Tornado Alley.

2011 was one of the worst tornado seasons in living memory,

with destruction on a scale rarely seen.

Did you see that?

The whole house blew apart.

Oh my god.

Oh my god.

In Joplin, Missouri, a tornado cut a six mile-wide path of damage, killing more than 160 people.

But what's really terrifying about tornadoes...

...is that though the science behind their power and operation is well known,

accurately predicting them remains tantalisingly out of reach.

Josh Wurman is an atmospheric scientist.

He and his team from the Centre for Severe Weather Research in Colorado...

...are investigating how thunderstorms produce tornadoes.

Tornadoes depend on the transition of seasons.

We need summery conditions at the surface,

a lot of warmth and humidity that gives us energy for thunderstorms.

But then we still need a wintertime jet stream,

we still need the strong jet stream...

...which is caused by the contrast of tropical temperatures and the still cold Arctic.

So we need that combination of conditions,

which only really exists during the transition season of spring.

The missing piece of the puzzle that...

...we're still actively researching is exactly how does storms form the tornadoes?

About three quarters of them don't.

But a quarter of them do,

and we have a very hard time predicting exactly which ones of those are going to make tornadoes,

when they're going to make tornadoes,

and which ones are going to make the strongest and most violent tornadoes.

To get answers to these questions,

Josh and his team need to record data from as many tornadoes as they can during the season.

To hunt down developing storms, Josh uses a Doppler radar scanner.

It can show him what's going on inside a thunderstorm,

giving him critical information about the conditions needed to trigger a tornado.

The Doppler radar is able to send a beam of microwaves out through a tornado...

...and make maps of where it's raining, how hard it's raining,

whether the winds are going strong,

whether they're weaker and which direction the winds are going,

and watch how a tornado is evolving.

As the Earth's tilt gradually points the northern hemisphere...

...further towards the Sun and spring progresses, tornado activity moves north.

But although Josh knows roughly where to look, finding individual storms is always difficult.

Then, after the team has driven over a thousand miles and have crossed six state lines,

Josh's radar detects a storm.

It's beginning to show the tell-tale movement of clouds.

The team has to move fast: tornadoes form and vanish very quickly.

We're watching some very strong rotation here,

this purple area here is winds about 150 kilometres towards the radar.

We're also watching this kind of swirl down here...

...which is associated with a rotation that's coming up the road towards us.

High above the Midwest, this thunderstorm has hit the jet stream, which has made it start to spin.

The storm becomes a giant rotating air mass, a super-cell.

Well, right now we're intercepting a super-cell...

...that is probably made from small, short-lived tornadoes.

Going on ahead, this big dark area is the core.

So we're basically going to penetrate through the core and see what's interesting.

Yeah, right now we're kind of right in the centre of that coiled part of it.

When the spinning column of air touches the ground,

a tornado is born.

At the tornado's rotating core is an area of intense low pressure which draws high pressure air towards it.

Normally this is an invisible process.

But a tornado sucks in dust and debris.

There we go, that's what it's about.

This is what all the effort has been for.

A huge tornado less than a mile ahead.

It's one of the most dramatic consequences of the Earth's tilt...

...and what can happen in the northern hemisphere when spring collides with winter.

In this case, the Earth's tilt had given birth to an F3 rated tornado,

with winds travelling at up to 200 miles per hour.

But less than half an hour after it first formed, the tornado dies.

It was just one of more than 1200 tornadoes...

...that have battered this region since the beginning of spring.

This year more than any year in recent memory,

we've seen how bad tornadoes can be.

We've seen tornadoes go through cities, we've had over 500 people killed so far.

If we can increase the accuracy of warnings, then people may have time to get to better shelter.

Tornadoes are not the only consequence of our planet's tilt...

...driving the onset of spring in the northern hemisphere.

The unstable atmosphere caused by warming air masses as they invade the north,

triggers other big weather events around the hemisphere.

Storms can cause heavy and sudden downpours, which can result in flash floods.

These occur when heavy rain hits ground already saturated with water.

Seasonal thunderstorms can also give birth to a phenomenon called a haboob,

like this one in Phoenix, Arizona.

These massive dust storms occur in arid regions when the leading edge of a thunderstorm collapses,

generating a superfast downdraught.

Falling winds kick up a wall of dust and sand in front of it.

As summer progresses,

the planet's tilt pushes the northern hemisphere even further towards the sun.

This brings even more solar energy to the warming land...

...and generates the biggest single weather event on the planet.

Oceanographer Helen Czerski...

...has come to Kerala on lndia's southern coast to witness the start of the monsoon.

We're on the edge, and the land - the big Indian land mass behind me,

the ocean's to the south,

and this is where the monsoon is set up.

Whats driving the monsoon is the difference between the land and the sea.

So the sun is beating down on this area of land,

especially at this time of year when the sun's almost directly overhead.

And the land heats up very, very quickly, whereas the ocean heats up much more slowly.

If you look at the sand here, the surface has been heated by the sun.

It's hot.

And if you dig down, you don't have to dig down very far,

even a little way down that sand's really cool, whereas this layer here is really, really hot.

And you can see that only a thin layer is being heated,

so all of that sun's energy that's being absorbed is going into this really thin layer...

...so it heats up very, very quickly.

So the sun heats the land dramatically, but not the ocean.

Here, the Indian Ocean is much cooler than the land.

The reason is the ocean requires much more of the sun's energy to heat it up.

The ocean is also relatively cool because to heat the surface,

you have to heat much more than just a thin layer.

What happens is that winds that blow across the surface of the ocean...

...generate turbulence which mixes that top layer.

So as soon as some water's been heated at the top it gets mixed down below.

As we enter summer, the land heats up fast, while the ocean lags further and further behind.

Now the air above the land becomes much hotter than the air above the ocean.

As the warmer air mass rises above the Indian subcontinent,

it creates a kind of vacuum behind it.

This draws in the cooler, denser air from the ocean to replace it.

Because of lndia's particular geography, the process happens on a huge scale.

It's a triangular peninsular with both wide hot plains and, crucially, a very long coastline.

This combination sets up a powerful and sustained movement of cooler ocean air towards land,

called the monsoon wind.

But what we think of with monsoons is not wind, but rain.

There's an enormous cloud overhead,

an enormous black cloud which has built 15 kilometres up into the troposphere,

and the first drops of rain are just starting to come down.

As the sun beats down on the ocean,

some of the water evaporates and is carried up into the atmosphere,

where it forms vast rainclouds.

To capture their formation, Helen has set up a series of time lapse cameras.

The monsoon winds drive the moisture-laden clouds across the land,

bringing with them a torrential downpour.

Eighty per cent of all lndia's rains arrive in this seasonal deluge.

This is the monsoon.

This is all the water that's been lifted up off the ocean in the past few clays...

...because of the heat and has been carried inland and is now falling on top of me.

The raindrops here are really, really large,

and you can see how they leave pockmarks in the sand because they're so big.

And the reason for that is in warm conditions like this...

...up in the clouds the droplets are joining together to make bigger drops.

The smaller drops actually stay up in the clouds,

they're being lifted out of the way...

...and all that falls are these massive drops that are covering me now.

The monsoon is an epic weather event of huge importance to the whole subcontinent...

...and the people who live here - over a billion of them.

Nowhere is this more important than here in the city of Udaipur in Rajasthan,

part of lndia's north-western corner.

Though it starts in May on the Kerala coast,

the life-giving waters of the monsoon don't arrive here until mid-July.

Sometimes they don't arrive at all.

Because of the importance of the annual rains,

the 72nd Maharana of Udaipur built Sajanghar, the monsoon palace.

Sajanghar provides a panoramic view of the clouds that herald the start of the monsoon,

which is still critical to the region today.

As the 72nd Maharana's descendant, Prince Lakshyaraj knows well.

The monsoons are very, very important for us, purely because everybody is dependent on water.

Obviously it makes a huge difference to the farmers.

It can show stability for the region, for crops, for tourism, for livelihood.

So it's very, very important.

Every drop of monsoon rain is so critical here,

and the prince's ancestors didn't stop at building a viewing platform,

but went on to build a network of artificial lakes to capture and store the precious monsoon rain.

Because when the earth tilts back the other way,

there are many months of dry weather with not a drop of rain.

We're very fortunate for the vision that the rulers then had of building a lot of lakes...

...in order to be able to store a lot of water for the people of Udaipur...

...and it's still ensuring the welfare of the people today.

Monsoon is not unique to India.

There are seasonal monsoon rains in Africa, Asia, Australia and even North America.

They are all generated by seasonal variations in surface heating...

...as different parts of the Earth tilt towards and away from the Sun.

When Thea and the young Earth collided billions of years ago,

it gave us the seasons, paving the way for some of the world's most spectacular events.

But that isn't quite the end of the story.

This is the Egyptian Sahara.

It's searingly hot and dry.

But it wasn't always like this.

Hidden in this lifeless landscape is remarkable evidence of how the planet's tilt...

...has had an impact on climate, transformed landscapes,

and profoundly altered the course of human history.

Geographer and Palaeo-climatologist Nick Drake...

...is travelling deep into the Sahara close to the Libyan border to discover...

...how, when and why the Saharan environment changed.

After three days of travelling, there's a gradual change in the landscape.

The dunes give way and hills and ravines begin to appear.

This is the vast limestone plateau known as the Gilf Kebir, or Great Barrier.

Hidden in this forbidding landscape are intriguing clues to this desert's past.

This is the Cave of Beasts, discovered in 2002.

It gives us an extraordinary glimpse into a very different Sahara.

Wow, what a vista of humans and animals we've got here.

We've got a lovely line of what appear to be antelopes.

This one look like it's running.

Oh, and here we've got a really nice one.

A nice giraffe there.

And over here what looks like a Barbary sheep, with these big curly horns.

Here we have a goat.

And these are quite interesting,

because the goats were introduced in the Sahara sometime between 6,000 and 7,000 years ago.

So it gives us a time frame for this rock art.

These remarkable paintings are up to 3,000 years older than the pyramids.

But it's the depictions of humans that are most revealing.

It looks these ones, a load of people here holding hands, some sort of ceremony maybe.

And here we've got further evidence of human activity.

Hand prints.

Someone has been here thousands of years ago, put their hand on this rock here.

And this person's roughly my size.

Look over here, children.

The hands are all intermediate sizes,

suggesting there's a whole community here living in this area that's now a desert.

Maybe they just wanted to say,

'I was here,' and leave their signature on the rocks.

Some of the human activity scenes suggest the presence of water.

And this is a lovely figure here.

Some people believe swimming.

And there's a line of these coming along here.

There's another excellent example there.

So what we've got here...

...is savannah animals, human activity, some of which suggests swimming.

Evidence of communities and over long periods of time,

which suggests a wet environment when they were living here in the past.

So where did the waters that sustained those people and animals come from'?

A day's travel away is the valley of Wadi Bakt.

Here there are clues that may help to resolve this great mystery.

Layers of sediment are a perfectly preserved historical record of the climate here.

And what they reveal is astonishing.

Okay, what we got here is layers of different sediment.

At the bottom here we've got sand.

This sand is much the same as the sand we see today.

It indicates a dry environment, sand blowing around and being deposited on the surface.

And then above this we've got this darker, sort of grey layer of clay.

The clay's been deposited by a river.

There's a river flowing down this valley here, depositing these sediments.

Well, this is evidence of the monsoon.

We've got seasonally wet, seasonally dry, seasonally wet, seasonally dry.

Some of them for longer periods of time where we get a really enhanced monsoon,

some shorter periods of time.

We may just get an annual wetting and drying.

And we know when that was.

These sediments have been dated through radio carbon dating,

and the sediments right down the bottom there date to about 10,000 years ago.

And the sediments up the top here date to 5,500 years ago.

This is 4,500 years of monsoon here in the Sahara, one of the driest places on Earth today.

Regular monsoon rains fed a lake in Wadi Bakt for 4,500 years,

so human swimmers are not so unlikely.

Well, what we've got here is a spectacular looking lake.

We've got these lake sediments stretching across much of this valley,

from where we can see.

We can see them...

...extending over here to the other side of the valley and the face of that escarpment.

So we've got a pretty big lake here.

We'd have had grasslands with scattered trees.

Around the edge of these lakes we'd have had marshes,

we'd have had hippopotamuses and crocodiles in those lakes.

We'd have had herds of antelopes and wildebeests roaming across the plains.

Just your average savannah, but stretching right the way across the Sahara.

Nick's research has revealed that the ancient African monsoon...

...created an entire network of rivers and lakes that crisscrossed the Sahara.

These waterways, some the size of the Caspian Sea,

supported communities of animals and people.

But what was it that brought the monsoon rains to the Sahara'?

More importantly, what made them cease?

The answer lies with the sun.

As with the Indian monsoon today, summer heating across the Sahara acted as a huge heat engine...

...drawing moisture in from the ocean and depositing it as rain across the continent.

And just as with the Indian monsoon, the more heating, the stronger the effect.

So during the time of the green Sahara,

temperatures must have been even hotter than today,

driving a monsoon system into the desert.

Scientists have linked these higher temperatures with changes to the Earth's angle of tilt.

Today the angle of tilt is 23.4 degrees,

but every 41,000 years that angle swings between 22 and 24 and a half degrees.

Back when the Sahara was green,

the Earth was close to its maximum tilt of 24 and a half degrees.

It may seem like a tiny shift,

but combined with small cyclical changes in the direction of the title and the shape of our orbit,

the results was the sun shone more intensely over the northern hemisphere...

...and it powered a Saharan monsoon.

Then, five and a half thousand years ago, the cycles changed.

The Earth's tilt decreased again, moving closer to what it is now.

The Saharan monsoon stopped, and very quickly the water and vegetation started to disappear.

The green savannah was transformed, leaving what we see today:

one of the world's largest deserts.

Recent research suggests that...

...the Saharan monsoons may have had an even greater impact on human civilisation.

Old lake sediments suggest there was another wet phase in the Sahara's history,

120,000 years ago.

This particular green Saharan episode is a really exciting one,

because it may well have controlled the migration of humans out of Africa.

Modern humans evolved in Africa around 200,000 years ago.

But how they migrated through the impassible Saharan desert...

...and out of Africa has long been a mystery.

A green Sahara 120,000 years ago may provide the answer.

It is at this time when we find the first evidence of modern humans...

...to the north of the Sahara, along the Mediterranean coast.

Presumably, these humans got across that green Sahara at that time.

And once they're up across the Sahara in the - in the - along the Mediterranean coast,

it's easy for them to just migrate out through the Levant and then into Europe,

which they did about 50,000 years ago.

It's remarkable to think that tiny variations in the Earth's tilt...

...may have led to one of the most fundamental movements in human history:

our exodus out of Africa.

And equally remarkable to think that because of the Earth's changing tilt,

there will come a time when the Sahara will be green again.

But not for another 15,000 years.

The seasonal cycles of the years are like Earth's heartbeat.

They give our planet its personality, range of climates and habitats.

They even provide the vast numbers of plant and animal species...

...that the ancients ascribed to mystical power or divine providence.

It's little wonder then that they celebrated the significant points in a year.

The twice-yearly equinoxes,

when equal periods of day and night mark the divisions between summer and winter.

And the solstices in winter and summer,

where the days reach their minimum and maximum durations.

June 21st is the longest day in the northern hemisphere,

the summer solstice.

Here, at Kom Ombo, near the ancient city of Aswan,

the sun worshipping Egyptians were able to celebrate their deity's finest hour in this magnificent temple.

And because of its location on the globe,

this shrine to the Sun is one of the most significant places to witness the solstice.

At noon the sun is directly overhead...

...and shines into this water well making it a perfect light well.

If we were to trace a line parallel with the equator from Aswan right around the globe,

it would be the furthest point north the midday sun can ever be directly overhead.

This is the Tropic of Cancer,

and because the Earth is tilted at 23.4 degrees from the vertical,

the Tropic of Cancer lies 23.4 degrees above the equator.

And the June solstice defines another significant line of latitude.

As the northern hemisphere points towards the Sun, the Arctic experiences 24 hours of sunlight.

On the solstice, the midnight sun reaches its fullest extent,

a line marked by the Arctic Circle, which is 23 and a half degrees from the North Pole.

It's shaped our destiny and informed our beliefs about ourselves and our place in the universe.

It's given us terrifying storms.

Oh my god.

It's showered us with benevolent seasonal rains.

It's driven the astounding diversity of life on Earth.

And even enabled the flourishing of our own species.

What was once thought to be mysterious...

...and beyond our comprehension turns out to be the simple truth of the Earth's tilt.

Our planet is what it is today because of the small matter of 23.4 degrees.

What it will be in the future will be in large part due to what that number becomes.